1. Field of the Invention
The invention concerns a cardiac pacemaker which, via an electrode implanted in the atrial and/or ventricular cavity, emits effective electric pulses aimed to keep cardiac contractions regular and constant.
2. Description of the Related Art
Currently, in the field of permanent or temporary cardiac pacing, pacemakers exist, all of which are characterized by an escape interval which determines a stimulation frequency that bears no relation to the sequence of natural electric pulses at the base of the heart's natural functioning and which make up the framework in which the pacemaker has to work.
Existing electrostimulators have an escape interval that is programmed to a fixed rate, usually selected within a range that guarantees a heart rate of between 70 and 80 beats per minute. Some pacemakers, of the so-called "Rate Responsive" (R.R.) type, adjust the frequency to meet the demands of increased cardiac load during movement or effort.
In this case the increased cardiac frequency corresponds to muscular activity discerned by strain occuring in piezoelectric sensors, by sensors which measure pulmonary ventilation and by other types of sensors. Other so-called "hysteresis" pacemakers progressed from a single to two escape intervals: one, longer interval for the initial triggering of the artificial pacemaker and the other, shorter and constant interval for subsequent stimulation intervals in the case of prolonged cardiac pacing. In order to favour the maintenance of the natural rhythm even more, this method was subsequently modified with the advent of the "search hysteresis" pacemaker. In this case the deactivation threshold of the artificial pacemaker is occasionally adjusted so as to favour the re-emergence of the natural rhythm. However, even with the application of hysteresis, the escape intervals are fixed. During a check-up it is possible to set them to different rates, but once memorized, they can no longer adjust automatically to adapt to the variations in the patient's heart rate.
With the introduction of the "rate responsive" function, the escape intervals are spread over a much wider range in an attempt to adapt to increases in the patient's metabolic demands. There still remains the fact that with both the "rate responsive" and the "hysteresis" equipped pacemakers, the escape interval is completely unconnected to natural electric stimuli which very often continue to be set off and appear, even though in an irregular and inefficient way.
Existing cardiac pacemakers therefore can interfere with the natural rhythm, nullifying the daily physiological variation of the sinusal drive, destroying the physiological triggering sequence between the atrium and the ventricle and provoking a serious cardio-circulatory imbalance termed "pacemaker syndrome".